Dryer preheater

ABSTRACT

A preheater for a clothes dryer or the like includes an elongated shell housing having a heat exchange chamber and an exhaust air plenum assembly with inlet orifice means to said chamber for the air to be heated extending substantially around the full periphery of the housing adjacent both ends. The plenum assembly includes an air intake chamber for the exhaust air from the dryer at one of the ends and an air discharge chamber for the cooled exhaust air at the other end. Baffles are provided at the ends of the housing defining said intake and discharge chambers and the heat exchange chamber therebetween. An output passage to feed preheated air to the dryer is positioned intermediate the ends such that the ratio of distance from the air intake means and the air exhaust means is approximately 4:1. The shell housing is preferably a polygon in cross section and the inlet orifice means includes first openings adjacent the air intake on each side and second openings adjacent the air exhaust on each side. The first openings are larger than the second openings by approximately two and one-half times.

1 1 Jan. 14, 1975 DRYER PREHEATER [76] Inventor: Herman E. Katterjohn, Jr., 1021 Kentucky Ave., Paducah, Ky, 42001 [22] Filed: Jan. 23, 1974 [21] Appl. No.: 435,652

Primary Examiner-Kenneth W. Sprague Assistant Examiner-James C. Yeung Attorney, Agent, or FirmLowe, King & Price [57] ABSTRACT A preheater for a clothes dryer or the like includes an elongated shell housing having a heat exchange cham ber and an exhaust air plenum assembly with inlet orifice means to said chamber for the: air to be heated extending substantially around the full periphery of the housing adjacent both ends. The plenum assembly includes an air intake chamber for the exhaust air from the dryer at one of the ends and an air discharge chamber for the cooled exhaust air at the other end. Baffles are provided at the ends of the housing defining said intake and discharge chambers and the heat exchange chamber therebetween. An output passage to feed preheated air to the dryer is positioned intermediate the ends such that the ratio of distance from the air intake means and the air exhaust means is approximately 4:1. The shell housing is preferably a polygon in cross section and the inlet orifice means includes first openings adjacent the air intake on each side and second openings adjacent the air exhaust on each side, The first openings are larger than the second openings by approximately two and one-half times.

6 Claims, 9 Drawing Figures PATENTED JAR I 41975 sum 10F 3 DRYER PREHEATER The present invention relates to preheaters for dryers or the like, and more particularly, to an improved structure for a preheater providing maximum heat transfer efficiency.

BACKGROUND OF THE INVENTION In the heat exchange art, there has been a substantial effort made by industry to improve the efficiency of devices in which heat is utilized to do useful work. One major area of inventive work has been directed to the conservation of heat energy by utilizing exhaust heat to preheat the incoming fluid, such as air or water, that is being used in any particular system. In the electrical power field for example, where giant boiler systems are utilized for feeding the required high enthalpy steam to the turbines that in turn drive the generators for producing electricity, engineers have realized a high level of efficiency of conserving energy in preheating the makeup water entering the system. The type of preheater that has become a standard for boilers has heated exhaust liquid intake means at one end, discharge means at the opposite end, and makeup water inlet and output passages adjacent the ends communicating with the heat exchange chamber. The flow of makeup water is counter to the exhaust liquid flow and the flows are separated by a conventional confining tube bundle comprising a plurality of tubes mounted by tube sheets at the ends.

In the related field of heating and using air, particularly in the environment where the useful work is for drying material, such as tobacco, similar preheating arrangements have been proposed. The patent to Zeun U.S. Pat. No. 2,033,169, issued Mar. 10, 1936 shows in FIG. a system for heating inlet air substantially as has been the practice in preheaters for heating makeup water for generation of steam for driving turbines. The inlet air enters at one end, passes through the tubes in the heat exchange chamber and is discharged through the other end. The exhaust heating medium enters through single openings at the ends, passes around the tubes and exits through single openings in the middle.

' Similar arrangements have been proposed for preheating air wherein the two air mediums are merely reversed, i.e., the inputs and outputs are arranged to allow 1 the inlet air to enter the heat exchange chamber at the ends, (2) the heated air to be outputted through an opening positioned intermediate the ends, and (3) the heating medium to pass through an exhaust air plenum assembly having intake and discharge means at the ends and a tube bundle therebetween with the medium passing through the interior of the tubes.

The preheater having the latter described arrange ment has been proposed for replacement of preheaters on prior art dryer systems, such as shown in the Solem U.S. Pat. No. 3,066,423, issued Dec. 4, 1962. This prior art system requires substantial extra space and relatively expensive structure as compared to the described system. Similarly, the proposed system represents substantial improvement over prior arrangements where a portion of the moist exhaust air is merely bled off from the exhaust and fed back to the dryer, not unlike the system shown in the Ross U.S. Pat. No. 1,365,790, issued Jan. 18, 1971. Likewise, the system is superior to the prior art arrangement where the exhaust heated air is utilized to heat fluids used in other locations within a laundry, such as suggested by the Friedman U.S. Pat. No. 3,050,867, issued Aug. 28, I962.

However, improvement over the basic arrangement shown in the Zeun patent and the proposed change described, has been identified in the area of gaining maximum heat transfer efficiency. This need has been polarized by the recent developments of fuel shortages throughout the world, and accordingly, the structure of the present invention has resulted from my increased reasearch and discoveries brought about by such need.

OBJECTIVES OF THE INVENTION Thus, it is one object of the present invention to provide an improved preheater for a dryer that provides maximum heat transfer efficiency.

It is another object of the present invention to provide a preheater wherein more complete scrubbing contact of incoming ambient air is provided on the surface area of all heat transfer tubes by transverse air flow and turbulence generated around the full surface area of the tubes.

It is still another object of the present invention to provide an improved preheater for a dryer for clothes or the like wherein the structure is maintained compact for ease of installation and manufacturing economy, while increasing the heat transfer efficiency to a maximum.

BRIEF DESCRIPTION OF THE INVENTION In accordance with the present disclosure, a preheater for a clothes dryer or the like includes an elongated shell housing having a heat exchange chamber and an exhaust air plenum assembly, which housing may be easily adapted for use with a standard clothes dryer. The air intake means for moisture laden exhaust air is at the bottom of the housing directly adjacent the standard exhaust orifice of the dryer and the air discharge means for the cooled moisture laden exhaust air is at the top for interconnection directly to the exhaust stack to release the residue to the atmosphere. Baffle means extend across the chamber defining the intake and discharge chambers and the heat exchange cham ber therebetween. Elongated heat exchange tubes provide the interface between the fluid mediums with an output passage to feed the preheated air to the dryer intermediate the ends of the housing.

In accordance with one important aspect of the present invention, inlet orifice means for the incoming ambient air is provided substantially around the full periphery of the shell housing adjacent the baffles at the ends, as described above. This innovation allows full or total scrubbing contact of the incoming ambient air with the end surface area of all tubes substantially upon entry into the heat exchange chamber. The entering air is subject to transverse air flow intersection in these end areas thus generating turbulence and enhancing the heat transfer by radiation and forced convection. The air thus heated moves toward the center of the shell housing and intersects at an intermediate position. Here the output passage is located and the intersection of the two air flows from the ends generates further turbulence and heat transfer efficiency.

Preferably, the shell housing is a polygon, such as a square, in cross section, and the inlet orifice means includes first openings adjacent the air intake means and second openings adjacent the air exhaust means. The

first openings are proportionally larger than the second openings in accordance with the distribution of the heat content in the plenum assembly. Thus, since the heat is greatest at the end adjacent the air intake means, the openings have a larger area at this end. Specifically, the discovery has been made that the total area of the first openings should exceed the second openings by approximately two and one-half times and should consist of separate openings on all of the sides of the housing. Also, at least one opening of each group should be diverse in size and location to enhance and maintain the turbulent flow. The intermediate position of the output passage has also been specifically located for maximum heat transfer efficiency, such that the ratio of distance of the output passage from the air intake means and the air discharge means is approximately 4:1.

Still other objects and advantages of the present invention will become readily apparent to those skilled in the art from the following detailed description, wherein I have shown and described only the preferred embodiment of the invention, simply by way of illustration of the best mode contemplated by me of carrying out my invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modification in various obvious respects, all without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an overall perspective view of a clothes dryer having a preheater constructed in accordance with the present invention installed thereon, looking from the rear of the dryer;

FIG. 2 is a right-hand side view taken with respect to FIG. 1 and partially in cross section to show the tube bundle within the heat exchange chamber and diagrammatic flow of the ambient air being heated;

FIG. 3 is a view of the back side of the preheater taken in accordance with the showing of FIG. 1;

FIG. 4 is a top plan view of the preheater;

FIG. 5 is a horizontal cross section of the preheater taken along line 5-5 of FIG. 2 and showing the preferred pattern of the tubes in the tube bundle;

FIG. 6 is a left-hand view of the preheater taken in accordance with FIG. 1;

FIG. 7 is a front view of the preheater taken in accordance with FIG. I;

FIG. 8 is an enlarged cross sectional view of the top end of the preheater showing the air discharge chamber, the adjacent structure of the exhaust air plenum assembly and the resulting air flow pattern; and

FIG. 9 is a similar enlarged showing of the air intake chamber with the adjacent plenum assembly and air flow pattern.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIGS. l-9 of the drawings, a more detailed description of the invention can be given in order to provide a fuller understanding thereof. A preheater 10, constructed in accordance with the principles of the present invention, is shown in FIG. 1 attached in working relationship to a dryer D for clothes or the like, which dryer may generally be of any type or style ofmanufacture. The preheater 10 is connected to the usual exhaust outlet of the dryer D through a conventional pipe elbow 11, which is thus operative to feed the moisture laden heated exhaust air to said preheater 10. At the opposite or upper end of the preheater 10, the cooled residual moisture laden air is released to the atmosphere through the exit pipe 12. The preheated air that is fed to the dryer D passes through transfer pipe 13 emerging from the front of the preheater 10 and enters the dryer D through a suitable opening in the top thereof. It should be noted at the outset that the preheater 10, as shown in FIG. 1, is a complete structure in and of itself fully adapted to perform the preheating function without any additional attachments or space, as has been required in such prior art arrangements as proposed in the Solem patent, mentioned above. Also, as will be seen below, the structure is not only compact but performs the function in a highly efficient manner and thus represents a significant advance in the state of the art.

Continuing now to consider the specific structure of the preheater 10, a shell housing 20 forms the body of the structure and is in the preferred embodiment, a polygon in cross section; specifically, a square in cross section. As illustrated in FIGS. 2 and 3, most or all of the outer skin of the housing 20 is preferably insulated with insulation sheets 21 to reduce the loss of heat from the unit. The interior of the housing 20 forms a heat exchange chamber C through which the ambient air passes while being heated, as will be more fully understood as the discussion below progresses. The chamber C is delineated by the four walls of the housing 20,

which may be designated as a front wall W a back wall w, as best shown in FIGS. 2, 3 and 7, and a right-hand wall W3 and a left-hand wall w.,, as best shown in FIGS. 2 and 6. The orientation of the walls w -w is viewed with respect to the showing in FIG. 1.

An exhaust air plenum assembly is provided in the preheater 10 to receive the moisture laden exhaust air, transmit the air through the heat exchange chamber C inside the tube bundle and then discharge the cooled moisture laden air to the atmosphere. The plenum assembly includes a first reducer pipe section 25 (see FIGS. 2 and 9) forming air intake chamber C by virtue of the interconnection to the elbow 11 (see FIG. 1). Extending across the housing 20 at the juncture of the main body with the reducer section 25 (see FIG. 9) is a baffle plate 26 thereby forming the boundary between the heat transfer chamber C and the air intake chamber C The baffle 26 is provided with a plurality of openings through which pass a plurality of elongated heat exchange tubes 27. In the preferred embodiment illustrated, there are nine tubes 27 that form the tube bundle, although it is to be understood that other numbers of tubes may be used. The tubes are supported by and sealed to the baffle 26 by rolled edges, as best shown in FIG. 9.

The opposite end of the shell housing 20 has attached thereto a second pipe reducer 28 that formsa similar air discharge chamber C as best shown in FIG. 8. A second baffle 29 forms the dividing barrier between the inside of the heat exchanger C and the air discharge chamber C so that there is also no direct communication between these chambers. The upper ends of the tubes 27 are supported and sealed by the same type of rolled edges as at the other end, as shown in FIG. 8. Both the first reducer 25 and the second reducer 28, and the corresponding baffles 26 and 29 are interconnected with the main body of the housing 20 in a simple manner by interlocking rolled and flared edges, and threaded fasteners, such as fastenersf and f as shown in FIGS. 9 and 8, respectively.

The exhaust air plenum assembly thus includes the intake chamber C the interior of all of the tubes 27 of the tube bundle and the air discharge chamber C The exhaust heat coming from the dryer D is thus forced through the tubes 27 so that the outer surface area of all of the tubes is exposed to the heat exchange chamber C within the housing 20. Ambient air flowing through the heat exchange chamber C is directed to an output passage P formed at a position intermediate the two ends of said housing 20. The output passage P is formed by a separate diverter box 30 and pipe attachment section 31, as best shown in FIGS. 2 and 4. The entry of the ambient air to the chamber C, the flow of the air through said chamber and the output of the air through the passage P is of critical importance to the improvement of the present invention and will now be described.

Thus, the housing 20 is provided with inlet orifice means, generally designated by the reference indicia, O 0 adjacent the air intake means or intake chamber C (FIG. 9) and the air discharge means or air discharge chamber C (see FIG. 8), respectively. The inlet orifice means 0 0 can be divided up into two distinct groups of openings as can be seen from the composite viewing of FIGS. 2, 3, 6 and 7 and in conjunction with FIG. 1.

With reference specifically to FIGS. 1 and 3, the orifice means 0 0 may include a first opening 40 adjacent the air intake means and a second opening 41 at the opposite end of the housing 20 adjacent the air discharge means. Through these openings 40, 41 ambient air is introduced into the heat exchange chamber C from all directions. This incoming air substantially im mediately comes into contact with the ends 27, 27" of the tubes 27, as best can be seen in'FIG. 3. By bringing air to be heated into the chamber C at both ends thereof, the incoming air is assured of coming into intimate contact with the full end surface area of all of the tubes 27. This assures that the total amount of heat being radiated and being transmitted by forced convection is available to be absorbed, and thus the maximum amount of heat may be transferred.

In addition to the openings 40, 41 on the back wall W2, the intake orifice means 0,, 0 include in each respective group, openings 42, 43 on the right-hand side W3 (FIG. 2), openings 44, 45 on the left-hand side w (FIG. 6), and openings 46, 47 on the front wall w (FIG. 7). In other words, the inlet orifice means 0 comprises openings 40, 42, 44, and 46 and the inlet orifice means 0 comprises the openings 41, 43, 45 and 47. Thus, it can be seen that the inlet orifice means extends substantially around the full periphery of the housing and communicates with the heat exchange chamber at both ends of the preheater 10 immediately adjacent the baffles 26, 29. This structure has been found to establish a condition where maximum heat transfer may be realized.

Although the full explanation of the theory behind the substantially improved results that flow from the design of the present invention may not be totally understood, it is clear that the air intake flow through the openings 40-47 is such as to allow full scrubbing contact of the incoming air with the end surface area of the ends 27', 27" of all of the tubes 27 substantially immediately upon entry of the air into the chamber C. This arrangement also provides a transverse air flow intersection around and between these ends 27', 27" so to generate turbulence and eddy currents substantially aiding the scrubbing action of the air on the tube surfaces. The heated air from the ends then moves toward the center of the housing 2f) continuing to come into intimate scrubbing contact with the remainder of the entire tube surface area. At the intermediate point between the two ends where the passage P for the out put to the dryer D is located, the air longitudinally intersects and generates further turbulence for maximum heat transfer in this area also. It is believed that the feedback turbulence from the intermediate point toward the two ends, and vice versa, further enhances the scrubbing action so that decidedly improved results of heat transfer efficiency are realized.

A further feature is the provision of at least one opening of each group of openings 40, 42, 44, 46 and 41, 43, 45, 47 to be diverse in size, shape and location so that the entry of air from all sides of the housing 20 is assured of being unbalanced and thus capable of enhancing and maintaining support of the desired turbulence. In the embodiment illustrated herein, the openings 40, 41 on the back wall w are larger and extend to the limits of the walls W3, w and the respective baffles 26, 29. The other three openings of each group may be substantially identical, as shown. Where openings are not positioned on all sides of the housing 20 and where the openings are of the same size and relative location, laminar air flow tends to be established and a proven lower efficiency is provided.

In accordance with another feature of the improved structure of the present invention, the first openings 40, 42, 44, 46 are proportionally larger than the second openings 41, 43, 45, 47. This is advantageous so that the major portion of the incoming ambient air is at taken at the end of the housing 20 adjacent the air intake section 25 where the maximum heat content exists. At the opposite or upper end of the housing 20, the air within the tubes 27 has been substantially cooled so that a lesser amount of heat transfer would be expected to take place. The greatest efficiency has been found to occur where the composite area of the first openings 40, 42, 44, 46 or the first orifice means 0 is two and one-half times the area of the second openings 41, 43, 45, 47 or the second orifice means 0 The intermediate position for the output passage P is also selected to give maximum heat exchange efficiency between the air in the plenum assembly and the air in the heat exchange chamber C. As viewed in FIG. 2, the distance from the intake chamber C, to the intermediate position is shown by the distance x, and the distance from the discharge chamber C is shown by the distance y. The distance x is desirably longer so that the air being heated in the lower area of the tubes 27 that has the highest heat content receives the maximum scrubbing, and thus the maximum absorption of the heat is assured. The most desirable ratio of distances xzy has been found to be approximately 4:1.

One example of dimensions of a prototype preheater unit of the invention suitable for a standard 30 lbs. natural gas-fired commercial clothes dryer is as follows:

Overall housing 20 (length) 67 inches Housing 20 (cross section) [2 inches, sq. Heat exchange chamber C (between baffles 26, 29, length) 5] inches Tubes 27 (diameter) 2.75 inches Air intake and discharge chambers C C (minimum diameter) 8 inches 5 inches X [2 inches 60 sq. inches 2 inches X [2 inches 24 sq. inches [.5 inches X 9 inches [3.5 sq. in. 2.375 inches X [0 inches 33.75 sq. in.

Opening 40 Opening 41 Openings 43, 45, 47 Openings 42, 44, 46 Output passage P [I II II ll (minimum diameter) 8 inches Distance x 4[ inches Distance y [0 inches A test has been conducted to establish the substantially increased drying efficiency of a selected dryer system that includes the preheater of the present invention. The test results tabulated below show that an increase in efficiency in terms of reduction of drying time over the normal ambient temperature ranges of approximately one-third can be expected, or to put it another way, without a preheater an increase in drying time of approximately 50% can be expected.

B Cooled exhaust air in preheater exit pipe [2 or in preheater discharge chamber C (FIG. 8);

C Preheated air to dryer D in preheater output passage P (FIG. 2) or in transfer pipe [3 (FIG. 1);

D Ambient intake air at intake openings 40-47 (0,,

O in preheating test mode; or at top intake with transfer pipe [3 removed in non-preheating test mode. In the non-preheating mode of the system (without preheater) temperatures C and D are identical.

TEST RESULTS Dryer Temperature and Time to Dry Load D With Preheater Without Preheater 0 A [70F A [70F B [60F 26 min. B [61F 4[ min. C [70F C 0 [0 A [70F A [70F B [65F 26 min. B [63F 4[ min. C= [70F C= 10F A [70F A [70F B [60F 23 min. B [70F 38 min. C [70F C 20F 30 A [70F A [70F B =165F 23 min. B [65F 35 min. C [70F C 30F 50 A [70F A [70F B [65F 23 min. B [68F 35 min. C [70F C 50F 80 A [78F A [70F B [60F 21 min. B [68F 32 min. C [70F C 80F 100 A [70F A [70F B [60F 21 min. B [70F 32 min. C [70F C [00F In summary, it can now be seen that an improved preheater structure has been provided in order to give maximum heat transfer efficiency, particularly when in use with a dryer for clothes or the like. Low cost manufacturing and minimum space requirements are also primary considerations that give the preheater 10 advantages over the described previous arrangements. The inlet orifice means 0 0 extends substantially around the full periphery of the two ends of the preheater 10 to generate improved internal turbulence action that is a major factor in assuring the most desirable heat transfer coefficient. Full scrubbing contact of the incoming turbulent air is allowed over the end surface area of all tubes 27 substantially immediately upon entry into the chamber C.

Making at least one of the first and second group of openings diverse in size and relative location assures that the air entering at the ends of the heat exchanger chamber C maintains the turbulent flow that is desired. The first openings 40, 42, 44, 46 are proportionally larger than the second openings 41, 43, 45, 47 so as to assure that the maximum air flow occurs over the portion of the tubes 27 having the highest heat content. The first openings preferably have a total area that exceeds the area of the second openings by approximately two and one-half times. The longitudinal intersection adjacent the output passage P (FIG. 2) generates further turbulence, and the feedback turbulence from this area, as well as the feedback turbulence from both ends, assures the maximum scrubbing contact along the remainder of the surface area of the tubes. This intermediate position of the output passage P is preferably such that the ratio x/y of the distance from the first openings and from the second openings is approximately 4:]. This causes the air entering said first openings to traverse the greatest distance within the heat exchange chamber C, and thus have the greatest time exposure to the heated surfaces of the tubes 27, and thereby still further enhance the heat transfer efficiency.

In this disclosure, there is shown and described only the preferred embodiment of the invention, but, as aforementioned, it is to be understood that the invention is capable of use in various other combinations and environment and is capable of changes or modifications within the scope of the inventive concept as expressed herein.

What is claimed is:

1. In a preheater for use with a dryer for clothes or the like including an elongated shell housing having a heat exchange chamber, and an exhaust air plenum assembly including air intake means for receiving moisture laden heated exhaust air from the dryer at one end of the housing, air discharge means for the cooled moisture laden exhaust air at the opposite end, baffle means across said chamber at each end defining said intake and discharge means and said chamber therebetween, and a plurality of elongated heat exchange tubes supported and sealed by said baffles serving to pass said exhaust air through said heat exchange chamber, an output passage extending from said chamber at a position intermediate said ends of said housing to feed preheated air to said dryer, the improvement comprising an inlet orifice means extending substantially around the full periphery of said housing and communicating with said chamber at both ends and adjacent said baffles for directly introducing ambient air to be heated, said orifice means being so positioned substantially around the periphery so as to allow full scrubbing contact of incoming ambient air with the end surface area of all tubes substantially immediately upon entry into said chamber and to provide transverse air flow intersection around the end surface area of said tubes in said chamber to generate turbulence, the heated air from both ends further coming into scrubbing contact with the remainder of the tube surface area and longitudinally intersecting at said intermediate position along said chamber to generate further turbulence, whereby maximum heat exchange efficiency between incoming ambient air and the full tube surface area is attained.

2. The preheater of claim 1 wherein said shell housing is a polygon in cross section and said inlet orifice means includes first and second openings on all the sides of said polygon, said first openings being adjacent the air intake means and the second openings being adjacent the air exhaust means of said plenum assembly.

3. The preheater ofclaim 2 wherein the first openings adjacent the air intake means are proportionally larger than the second openings adjacent the air discharge means in accordance with the distribution of heat content in said plenum assembly.

4. The preheater of claim 3 wherein the total area of said first openings exceed said second openings by approximately two and one-half times.

5. The preheater of claim 3 wherein the intermediate position of said output passage is such that the ratio of the distance of said position from said air intake means and from said air exhaust means of said plenum assem bly is approximately 4:1.

6. The preheater of claim 2 wherein one of said first and second openings is diverse in size and relative location to assure maintenance of turbulent flow. 

1. In a preheater for use with a dryer for clothes or the like including an elongated shell housing having a heat exchange chamber, and an exhaust air plenum assembly including air intake means for receiving moisture laden heated exhaust air from the dryer at one end of the housing, air discharge means for the cooled moisture laden exhaust air at the opposite end, baffle means across said chamber at each end defining said intake and discharge means and said chamber therebetween, and a plurality of elongated heat exchange tubes supported and sealed by said baffles serving to pass said exhaust air through said heat exchange chamber, an output passage extending from said chamber at a position intermediate said ends of said housing to feed preheated air to said dryer, the improvement comprising an inlet orifice means extending substantially around the full periphery of said housing and communicating with said chamber at both ends and adjacent said baffles for directly introducing ambient air to be heated, said orifice means being so positioned substantially around the periphery so as to allow full scrubbing contact of incoming ambient air with the end surface area of all tubes substantially immediately upon entry into said chamber and to provide transverse air flow intersection around the end surface area of said tubes in said chamber to generate turbulence, the heated air from both ends further coming into scrubbing contact with the remainder of the tube surface area and longitudinally intersecting at said intermediate position along said chamber to generate further turbulence, whereby maximum heat exchange efficiency between incoming ambient air and the full tube surface area is attained.
 2. The preheater of claim 1 wherein said shell housing is a polygon in cross section and said inlet orifice means includes first and second openings on all the sides of said polygon, said first openings being adjacent the air intake means and the second openings being adjacent the air exhaust means of said plenum assembly.
 3. The preheater of claim 2 wherein the first openings adjacent the air intake means are proportionally larger than the second openings adjacent the air discharge means in accordance with the distribution of heat content in said plenum assembly.
 4. The preheater of claim 3 wherein the total area of said first openings exceed said second openings by approximately two and one-half times.
 5. The preheater of claim 3 wherein the intermediate position of said output passage is such that the ratio of the distance of said position from said air intake means and from said air exhaust means of said plenum assembly is approximately 4:1.
 6. The preheater of claim 2 wherein one of said first and second openings is diverse in size and relative location to assure maintenance of turbulent flow. 